A transmission line may be terminated with an impedance that is matched to a characteristic impedance of the transmission line. However, if the termination impedance is in a variable environment, the impedance may vary undesirably. For example, in the case of the termination impedance on a processor die, the heat generated by the processor may affect the impedance. It has therefore been proposed to terminate transmission lines leading onto the die with controllably variable impedances, and to calibrate the on-die termination impedances by comparison with a standard impedance in a more stable environment.